In recent years, water-absorbent resin has been widely used in various fields, including hygienic materials, such as disposable diapers and sanitary napkins, agricultural and horticultural materials, such as water-retaining agents and soil conditioners, and industrial materials, such as water-blocking agents and agents for preventing dew condensation. Of these, water-absorbent resin is frequently used in, in particular, hygienic materials, such as disposable diapers and sanitary napkins.
As the water-absorbent resin, for example, hydrolysates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylate graft copolymers, saponified products of vinyl acetate-acrylic ester copolymers, and crosslinked products of partially neutralized acrylic acid polymers are known.
The water-absorbent resin is mainly produced by subjecting a water-soluble ethylenic unsaturated monomer to reversed-phase suspension polymerization or aqueous solution polymerization. Of these methods, the reversed-phase suspension polymerization uses an organic solvent as a dispersion medium. Therefore, the environmental impact, such as the emission of organic solvent to the outside of the system, e.g., to the air, must be taken into consideration. Further, for example, provision of an organic solvent recovery system is required to prevent organic solvent emissions caused by the high heat of polymerization or a rapid reaction.
Further, in hygienic materials, such as disposable diapers, the water-absorbent resin is required to have, as a desirable property, a high water-absorption capacity under a load to reduce the amount of re-wetting when pressure is applied after the absorption of a fluid, in addition to having a high water-retention capacity. Further, having a small amount of water soluble component is also an important requirement for preventing a viscous fluid from sticking to the skin. However, it is difficult for known methods for producing a water-absorbent resin to sufficiently achieve a balance among these properties.
In particular, in terms of a water-absorbent resin, the water-retention capacity and the water-absorption capacity under a load tend to oppose each other. Generally, in order to obtain a high water-retention capacity, the crosslinking density of the water-absorbent resin must be lowered. However, a reduction in crosslinking density lowers the gel strength, resulting in a reduction in the water-absorption capacity under a load. Further, a reduction in the crosslinking density of a water-absorbent resin increases non-crosslinked components. Therefore, when the resin is brought into contact with a fluid, the water soluble component is likely to be dissolved. As a result, when the resin is used in a hygienic material, etc., a rash may be caused due to the water soluble component.
As a technique for improving the properties of water-absorbent resin, a method for performing reversed-phase suspension polymerization using a specific amount of a specific surfactant (see Patent Literature 1); a method for performing reversed-phase suspension polymerization in two or more steps (see Patent Literature 2); a method for performing reversed-phase suspension polymerization using a specific amount of a persulfate as a polymerization initiator (see Patent Literature 3); and a method for using a water-soluble azo initiator for radical polymerization in the presence of a specific internal-crosslinking agent (see Patent Literature 4), and the like, are proposed.
However, the water-absorbent resins produced by the methods disclosed in Patent Literature 1 to 3 cannot sufficiently satisfy all of the properties, i.e., a high water-retention capacity, a high water-absorption capacity under a load, and a small amount of water soluble component. Further, in the reversed-phase suspension polymerization disclosed in the Examples of Patent Literature 4, a water-soluble azo initiator for radical polymerization is used in a large amount, which causes heat generation or a rapid reaction, resulting in the organic solvent being released to the outside of the reaction system; thus, the environmental impact is not considered therein.